1. Field of the Invention
The present invention relates to a swash plate type hydraulic actuator comprising a casing, cylinders supported for rotation on the casing and provided with a plurality of cylinder bores arranged on a circle. An annular inner oil passage is formed in the inner circumference thereof. An annular outer oil passage is formed in the outer circumference thereof. A plurality of radial valve holes extend between the annular inner oil passage and the annular outer oil passage and respectively having ports opening into the cylinder bores. Plungers are slidably fitted respectively in the cylinder bores. Swash plates and swash plate holders holding the swash plates, respectively, are provided so that the swash plates are in contact with one end of each of the plungers projecting from the cylinders. Distributing valves are slidably fitted in the valve holes so as to connect the ports alternately to the inner oil passage and the outer oil passage. Eccentric rings are held on the swash plate holders so as to be in contact with the outer ends of the distributing valves. A hydromechanical continuously variable speed transmission includes the swash plate-type hydraulic actuator. A method is disclosed for controlling the hydromechanical continuously variable speed transmission.
2. Description of Background Art
The aforesaid apparatus has been known and has been published in, for example, Japanese Patent Laid-open (Kokai) No. 3-162972.
The aforesaid prior art hydraulic continuously variable speed transmission comprises a fixed displacement swash plate hydraulic pump and a variable displacement swash plate type hydraulic motor. The displacement of the swash plate type hydraulic motor is changed by changing the inclination of the swash plate to change the stroke of the plungers. Since the swash plate of the hydraulic pump rotates and a complicated mechanism is necessary for changing the inclination of the rotating swash plate, only the hydraulic motor is of a variable displacement type. The displacement of the hydraulic motor must be increased to widen the range of the transmission gear ratio, which unavoidably increases the size and weight of the continuously variable speed transmission. Therefore, it has been desired to make the displacement variable whether or not the inclination of the swash plate is variable.